Novel method of preparing secondary battery

ABSTRACT

A method of preparing a secondary battery which includes pre-lithiating an electrode assembly which includes an electrode structure including a plurality of electrodes and a plurality of separators, and a metal substrate. The plurality of electrodes and the plurality of separators are alternatingly, stacked. The metal substrate is present on an outermost surface of the electrode structure in a direction in which the electrode and the separator are stacked. Each positive electrode and negative electrode are spaced apart from each other with one separator of the plurality of separators disposed therebetween. The pre-lithiating includes applying a first current by electrically connecting one of the plurality of positive electrodes and one of the plurality of negative electrodes, and applying a second current by electrically connecting the metal substrate and one of the plurality of positive electrodes, after applying the first current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Patent Application No.10-2020-0007192, filed on Jan. 20, 2020, the disclosure of which isincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a novel method of preparing a secondarybattery in which, during pre-lithiation of the secondary battery, aftera positive electrode and a negative electrode are connected to transferlithium to the negative electrode, the positive electrode and a lithiumlayer are connected to transfer lithium to the positive electrode.

BACKGROUND ART

Recently, with the rapid spread of electronic devices using batteries,such as mobile phones, notebook computers, and electric vehicles, demandfor secondary batteries with relatively high capacity as well as smallsize and lightweight has been rapidly increased. Particularly, since alithium secondary battery is lightweight and has high energy density,the lithium secondary battery is in the spotlight as a driving powersource for portable devices. Accordingly, research and developmentefforts for improving the performance of the lithium secondary batteryhave been actively conducted.

In general, the lithium secondary battery includes a positive electrode,a negative electrode, a separator disposed between the positiveelectrode and the negative electrode, an electrolyte, and an organicsolvent. Also, with respect to the positive electrode and the negativeelectrode, an active material layer including a positive electrodeactive material or a negative electrode active material may be formed ona current collector. A lithium-containing metal oxide, such as LiCoO₂and LiMn₂O₄, is generally used as the positive electrode active materialin the positive electrode, and, accordingly, a carbon-based activematerial or silicon-based active material containing no lithium is usedas the negative electrode active material in the negative electrode.

Particularly, pure silicon among the negative electrode active materialsis attracting attention in terms of having a capacity approximately 10times higher than that of the carbon-based active material, and isadvantageous in that high energy density may be achieved even with athin electrode due to its high capacity. However, the silicon has alimitation in its use due to low initial efficiency and excessive volumeexpansion according to charge and discharge.

In order to address the low initial efficiency of the silicon, processesof intercalating lithium into the negative electrode in advance(pre-lithiation) have been used. Among these processes, after preparingan electrode assembly, a method of supplying lithium to the negativeelectrode by connecting the negative electrode and lithium metal isused. However, this method reduces processability because it takes a lotof time to reach a desired pre-lithiation level, and amounts of lithiumions intercalated into the negative electrodes in the electrode assemblyare significantly different from each other. Accordingly, capacity andlife characteristics of the battery may be degraded.

In the present invention, a new method of preparing a secondary battery,in which pre-lithiation may be performed at a faster rate and lithiumions may be uniformly intercalated into negative electrodes, isintroduced.

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides an improvement inpreparation processability of a battery by improving a pre-lithiationrate and an improvement in capacity and life (cycle) characteristics byuniformly intercalating lithium ions into negative electrodes.

Technical Solution

According to an aspect of the present invention, there is provided amethod of preparing a secondary battery which includes: pre-lithiatingan electrode assembly which includes an electrode structure including aplurality of electrodes and a plurality of separators; and a metalsubstrate, wherein the electrode and the separator are alternatinglystacked, the metal substrate is disposed on an outermost surface of theelectrode structure in a direction in which the electrode and theseparator are stacked, the electrode includes a negative electrodeincluding a negative electrode active material layer; and a positiveelectrode including a positive electrode active material layer, and thepositive electrode and the negative electrode are spaced apart from eachother with the separator disposed therebetween, wherein thepre-lithiating includes a first step of applying a first current byelectrically connecting the positive electrode and the negativeelectrode; and a second step of applying a second current byelectrically connecting the metal substrate and the positive electrode,after the first step.

Advantageous Effects

According to the present invention, since a positive electrode and anegative electrode are electrically connected to intercalate lithiumions from the positive electrode into the negative electrode in anelectrode assembly and the positive electrode and a metal substrateincluding a lithium layer are then connected to transfer the lithiumions to the positive electrode, a pre-lithiation rate may be improved bya large potential difference between the positive electrode and thelithium layer. Accordingly, preparation processability of a battery maybe improved. Also, in a case in which the lithium ions are intercalatedinto the negative electrode by the above manner, since the lithium ionsmay be uniformly intercalated into the negative electrodes, capacity andlife (cycle) characteristics of the battery may be improved anddistortion and disconnection of the battery may be improved becausedegrees of expansions in thickness of the negative electrodes in theelectrode assembly are uniform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view for explaining a first step in a method ofpreparing a secondary battery according to an embodiment of the presentinvention;

FIG. 2 is a schematic view for explaining a second step in the method ofpreparing a secondary battery according to the embodiment of the presentinvention;

FIG. 3 is a schematic view for explaining an electrode in the method ofpreparing a secondary battery according to the embodiment of the presentinvention;

FIG. 4 is a graph illustrating a change in voltage during a process ofpre-lithiating each of preliminary batteries of (A) Example 1 and (B)Comparative Examples 1; and

FIG. 5 is a graph illustrating an increase in thickness of five negativeelectrodes in each of electrode assemblies of (A) Example 1 and (B)Comparative Example 1 after the pre-lithiation process.

MODE FOR CARRYING OUT THE INVENTION

It will be understood that words or terms used in the specification andclaims shall not be interpreted as the meaning defined in commonly useddictionaries, and it will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting of thepresent invention. In the specification, the terms of a singular formmay include plural forms unless referred to the contrary.

It will be further understood that the terms “include,” “comprise,” or“have” when used in this specification, specify the presence of statedfeatures, numbers, steps, elements, or combinations thereof, but do notpreclude the presence or addition of one or more other features,numbers, steps, elements, or combinations thereof.

The expression “average particle diameter (D₅₀)” in the presentspecification may be defined as a particle diameter at a cumulativevolume of 50% in a particle size distribution curve. The averageparticle diameter (D₅₀), for example, may be measured by using a laserdiffraction method. The laser diffraction method may generally measure aparticle diameter ranging from a submicron level to a few mm and mayobtain highly repeatable and high-resolution results.

Hereinafter, the present invention will be described in detail.

<Method of Preparing Secondary Battery>

A method of preparing a secondary battery according to an embodiment ofthe present invention includes: pre-lithiating an electrode assemblywhich includes an electrode structure including a plurality ofelectrodes and a plurality of separators; and a metal substrate, whereinthe plurality of electrodes and the plurality of separators arealternatingly stacked, the metal substrate is present on an outermostsurface of the electrode structure in a direction in which the electrodeand the separator are stacked, the plurality of electrodes includesnegative electrodes, each including negative electrode active materiallayer, and positive electrodes, each including a positive electrodeactive material layer, and each positive electrode and negativeelectrode are spaced apart from each other with one separator of theplurality of separators disposed therebetween, wherein thepre-lithiating may include a first step of applying a first current byelectrically connecting one of the plurality of positive electrodes andone of the plurality of negative electrodes; and a second step ofapplying a second current by electrically connecting the metal substrateand one of the plurality of positive electrodes, after the first step ofapplying the first current.

Typically, pre-lithiation has been performed by supplying lithium ionsto a negative electrode simply by connecting the negative electrode anda metal substrate, but this method has a limitation in that it takes along time to complete the pre-lithiation due to a low potentialdifference between the negative electrode and the metal substrate(including a lithium layer). Also, since distances between the metalsubstrate and the negative electrodes in an electrode assembly are alldifferent, thicknesses of the negative electrodes in the electrodeassembly are not uniform after the pre-lithiation is completed, andthus, disconnection may occur due to a severe degree of distortion of abattery.

Alternatively, the method of preparing a secondary battery according tothe embodiment of the present invention is characterized in that thefirst step and the second step are sequentially performed. In the firststep, lithium ions are rapidly supplied from the positive electrode tothe negative electrode due to a large potential difference between thepositive electrode and the negative electrode. Thereafter, lithium ionsare supplied from the metal substrate to the positive electrode throughthe second step. In this case, a pre-lithiation rate may be improved bya large potential difference between the positive electrode and themetal substrate (including a lithium layer). Accordingly, preparationprocessability of a battery may be improved. Also, in a case in whichthe lithium ions are intercalated into the negative electrode by theabove manner, since the lithium ions may be uniformly intercalated intothe negative electrodes and degrees of expansions in thickness of thenegative electrodes in the electrode assembly are uniform, distortionand disconnection of the battery may be suppressed.

Referring to FIG. 1 and FIG. 2 , an electrode assembly 400 may includean electrode structure 300 and a metal substrate 220.

1. Electrode Structure

The electrode structure 300 may include electrodes 110 and 120 and aseparator 130. The electrodes 110 and 120 may be plural, and theseparator 130 may be plural. The electrodes 110 and 120 and theseparator 130 may be alternatingly stacked.

(1) Electrode

The electrodes 110 and 120 may include a negative electrode 110 and apositive electrode 120. The positive electrode 120 and the negativeelectrode 110 may be spaced apart from each other with the separator 130disposed therebetween. Specifically, the separator 130 may be disposedon the negative electrode 110 while being in contact with the negativeelectrode 110, and the positive electrode 120 is disposed on theseparator 130. Accordingly, the electrodes 110 and 120 and the separator130 may be alternatingly stacked.

The electrode may include current collectors 111 and 121. The currentcollectors 111 and 121 may be included in the negative electrode 110 andthe positive electrode 120, respectively.

The current collectors 111 and 121 are not particularly limited as longas they have high conductivity without causing adverse chemical changesin the battery. Specifically, copper, stainless steel, aluminum, nickel,titanium, fired carbon, copper or stainless steel that issurface-treated with one of carbon, nickel, titanium, silver, or thelike, and an aluminum-cadmium alloy may be used as the currentcollectors 111 and 121. The current collectors 111 and 121 may typicallyhave a thickness of 3 μm to 500 μm. Microscopic irregularities may beformed on surfaces of the current collectors 111 and 121 to improveadhesion with the negative electrode active material layer or thepositive electrode active material layer. For example, the currentcollectors 111 and 121 may be used in various shapes such as that of afilm, a sheet, a foil, a net, a porous body, a foam body, a non-wovenfabric body, and the like.

Furthermore, the current collector may include a primer layer. Theprimer layer constitutes the surface of the current collector.Specifically, the current collector may include a substrate, such ascopper, stainless steel, aluminum, nickel, titanium, fired carbon,copper or stainless steel that is surface-treated with one of carbon,nickel, titanium, silver, or the like, and an aluminum-cadmium alloy,and the primer layer disposed on the substrate. Adhesion between thecurrent collector and the negative electrode active material layer andadhesion between the current collector and the positive electrode activematerial layer may be improved by the primer layer. The primer layer mayinclude a conductive material and a polymer. The conductive material mayinclude any one selected from the group consisting of carbon black,acetylene black, Ketjen black, carbon fibers, carbon nanotubes, andgraphene, or a mixture of two or more thereof. The polymer may be anyone selected from the group consisting of polyvinylidene fluoride(PVDF), polyvinylidene fluoride-co-hexafluoro propylene, polyvinylidenefluoride-co-trichloroethylene, polybutyl acrylate, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate,polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate,cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol,cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methylcellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadienecopolymer, polyimide, and polyamide-imide, or a mixture of two or morethereof.

The negative electrode 110 may include a negative electrode activematerial layer 112. Specifically, the negative electrode active materiallayer 112 may be disposed on the current collector 111, and, morespecifically, the negative electrode active material layer 112 may bedisposed on one or both surfaces of the current collector 111.

The negative electrode active material layer 112 may include a negativeelectrode active material.

The negative electrode active material may include silicon, and may bespecifically formed of silicon.

The silicon is a particle formed of silicon (Si) and may includeso-called “pure silicon”. The pure silicon is advantageous in that itscapacity is about 2.5 to 3 times higher than that of silicon oxide (forexample, SiO_(x) (0<x<2)), but, since a degree of volumeexpansion/contraction of the silicon is much larger than that of thesilicon oxide, it is more difficult to commercialize the silicon.However, with respect to the present invention, since the battery is setto be operated only in a region where the volume expansion of thesilicon is not excessive through a pre-lithiation process, a problem ofdegradation of life characteristics of the battery due to the use of thesilicon may be minimized. Also, advantages, such as high capacity, highenergy density, and high rate capability, of the silicon may be morepreferably achieved.

The silicon may be silicon particles, silicon nanowires, or poroussilicon, but is not limited thereto.

An average particle diameter (D₅₀) of the silicon may be in a range of0.01 μm to 50 μm, for example, 0.1 μm to 10 μm. In a case in which theabove range is satisfied, an improvement in the life characteristics bythe pre-lithiation process may be more effectively performed.

The silicon may be included in an amount of 50 wt % to 90 wt %, forexample, 60 wt % to 80 wt % in the negative electrode active materiallayer 112. In a case in which the above range is satisfied, capacity andenergy density of the battery may be increased, and negative electrodeadhesion (resistance to exfoliation of the negative electrode activematerial from the negative electrode) may be maintained after thepre-lithiation process.

The negative electrode active material layer 112 may further include anegative electrode binder and/or a negative electrode conductive agenttogether with the negative electrode active material.

The negative electrode binder may be used to improve adhesion betweenthe negative electrode active material layer and a negative electrodecollector to be described later, or to improve adhesion between thesilicon particles.

Specifically, the negative electrode binder may include polyvinylidenefluoride (PVDF), polyvinylidene fluoride-co-hexafluoro propylene(PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene,polypropylene, an ethylene-propylene-diene polymer (EPDM), asulfonated-EPDM, a styrene-butadiene rubber (SBR), a fluoro rubber, andvarious copolymers thereof, and any one thereof or a mixture of two ormore thereof may be used.

The negative electrode conductive agent may be used to assist andimprove conductivity in the secondary battery, and is not particularlylimited as long as it has conductivity without causing adverse chemicalchanges. Specifically, the negative electrode conductive agent mayinclude at least one selected from the group consisting of graphite suchas natural graphite and artificial graphite; carbon black such asacetylene black, Ketjen black, channel black, furnace black, lamp black,and thermal black; conductive fibers such as carbon fibers or metalfibers; conductive tubes such as carbon nanotubes; fluorocarbon; metalpowders such as aluminum powder and nickel powder; conductive whiskerssuch as zinc oxide whiskers and potassium titanate whiskers; conductivemetal oxide such as titanium oxide; and polyphenylene derivatives, andmay preferably include carbon black in terms of achieving highconductivity.

A loading amount of the negative electrode active material layer 112 maybe in a range of 2 mAh/cm² to 50 mAh/cm², for example, 5 mAh/cm² to 25mAh/cm². In a case in which the above range is satisfied, the lifecharacteristics of the battery may be improved while the energy densityof the battery is highly maintained.

The positive electrode 120 may include a positive electrode activematerial layer 122.

The positive electrode active material layer 122 may include a positiveelectrode active material.

The positive electrode active material may include at least one selectedfrom the group consisting of LiMn₂O₄, Li(Ni_(p)Co_(q)Mn_(r)Ma_(s))O₂(0≤p≤1, 0≤q≤1, 0≤r≤1, 0≤s<1, p+q+r+s=1, Ma is at least one selected fromthe group consisting of aluminum (Al), iron (Fe), vanadium (V), chromium(Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum(Mo)), LiMbO₂ (Mb is at least one selected from the group consisting ofAl, Fe, V, Cr, Ti, Ta, Mg, and Mo), and LiFePO₄.

The positive electrode active material may be included in an amount of80 wt % to 99.9 wt % and may be specifically included in an amount of 90wt % to 99 wt % in the positive electrode active material layer 122. Ina case in which the above range is satisfied, the life characteristicsof the battery may be improved by improving positive electrode adhesionwhile the energy density of the battery is highly maintained.

The positive electrode active material layer 122 may further include apositive electrode binder and/or a positive electrode conductive agenttogether with the positive electrode active material.

The positive electrode binder may be used to improve adhesion betweenthe positive electrode active material layer and a positive electrodecollector to be described later, or to improve adhesion between thepositive electrode active material particles.

Specifically, the positive electrode binder may include polyvinylidenefluoride (PVDF), polyvinylidene fluoride-co-hexafluoro propylene(PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose,polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene,polypropylene, an ethylene-propylene-diene polymer (EPDM), asulfonated-EPDM, a styrene-butadiene rubber (SBR), a fluoro rubber, andvarious copolymers thereof, and any one thereof or a mixture of two ormore thereof may be used.

The positive electrode conductive agent may be used to assist andimprove conductivity in the secondary battery, and is not particularlylimited as long as it has conductivity without causing adverse chemicalchanges. Specifically, the positive electrode conductive agent mayinclude at least one selected from the group consisting of graphite suchas natural graphite and artificial graphite; carbon black such asacetylene black, Ketjen black, channel black, furnace black, lamp black,and thermal black; conductive fibers such as carbon fibers or metalfibers; conductive tubes such as carbon nanotubes; fluorocarbon; metalpowders such as aluminum powder and nickel powder; conductive whiskerssuch as zinc oxide whiskers and potassium titanate whiskers; conductivemetal oxide such as titanium oxide; and polyphenylene derivatives, andmay preferably include carbon black in terms of achieving highconductivity.

A loading amount of the positive electrode active material layer 122 maybe in a range of 1 mAh/cm² to 10 mAh/cm², for example, 2 mAh/cm² to 7mAh/cm². In a case in which the above range is satisfied, since theenergy density of the battery may be increased and an N/P ratio of thenegative electrode and the positive electrode may be adjusted to anappropriate level, the life characteristics of the battery may befurther improved.

The electrodes 110 and 120 may include holes 110′ and 120′. The holes110′ and 120′ may include at least one of the hole 110′ included in thenegative electrode 110 and the hole 120′ included in the positiveelectrode 120. In other words, the plurality of electrodes compriseholes penetrating the individual electrodes.

The holes 110′ and 120′ may penetrate the electrode. Specifically, withrespect to the negative electrode 110, the hole 110′ may penetrate boththe negative active material layer 112 and the current collector 111.With respect to the positive electrode 120, the hole 120′ may penetrateboth the positive electrode active material layer 122 and the currentcollector 121.

The holes 110′ and 120′ act as a passage through which the lithium maysmoothly reach the negative electrode 110 in the electrode structure300. Accordingly, since the pre-lithiation process may be smoothlyperformed by the holes 110′ and 120′, the battery is operated only in aregion where the volume expansion of the silicon is not excessive, andthus, the life characteristics of the battery may be improved.Furthermore, since the hole 110′ included in the negative electrode 110may play a role in minimizing generation of excessive stress due to thevolume expansion of the silicon, the life characteristics of the batterymay be further improved.

The holes 110′ and 120′ may be formed in a cylindrical shape, a squarecolumn shape, or a triangular column shape, and may be specificallyformed in a cylindrical shape.

The holes 110′ and 120′ may have a diameter of 1 μm to 100 μm, forexample, 5 μm to 30 μm (see FIG. 3 ). Herein, the diameter maycorrespond to a diameter of openings 110′a and 120′a of the holes 110′and 120′ formed on surfaces of the electrodes 110 and 120. In a case inwhich the above range is satisfied, movement of the lithium is smooth,and, particularly, when using the silicon, damage to the electrode dueto the volume expansion of the silicon may be prevented during thepre-lithiation process.

A ratio of an area of the surface of the electrodes 110 and 120 to anarea occupied by the openings 110′a and 120′a of the holes 110′ and 120′on the surface of the electrodes 110 and 120 may be in a range of99.9:0.1 to 80:20, and may be specifically in a range of 99.5:0.5 to90:10. In a case in which the above range is satisfied, there is aneffect of facilitating the movement of the lithium while suppressingstructural collapse of the electrode. Herein, the area of the surface ofthe electrodes 110 and 120 means an area excluding the area occupied bythe openings 110′a and 120′a of the holes 110′ and 120′.

The holes 110′ and 120′ may be plural, and a spacing between theplurality of holes 110′ and 120′ may be in a range of 2 μm to 3,000 μm,and may be specifically in a range of 15 μm to 400 μm. In a case inwhich the above range is satisfied, since a material transfer distancein a direction parallel to the surface of the electrode is decreased,uniform pre-lithiation may be achieved.

(2) Separator

The separator 130 separates the negative electrode and the positiveelectrode and provides a movement path of lithium ions, wherein anyseparator may be used as the separator without particular limitation aslong as it is typically used in a secondary battery. Specifically, aporous polymer film, for example, a porous polymer film prepared from apolyolefin-based polymer, such as an ethylene homopolymer, a propylenehomopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer,and an ethylene/methacrylate copolymer, or a laminated structure havingtwo or more layers thereof may be used as the separator. Also, a typicalporous nonwoven fabric, for example, a nonwoven fabric formed of highmelting point glass fibers or polyethylene terephthalate fibers may beused. Furthermore, a coated separator including a ceramic component or apolymer material may be used to secure heat resistance or mechanicalstrength, and the separator having a single layer or multilayerstructure may be selectively used.

2. Metal Substrate

The metal substrate 220 may be disposed on the electrode structure 300,and, specifically, the metal substrate 220 may be in contact with theelectrode structure 300. The metal substrate 220 may be disposed on anoutermost surface 300 a or 300 b of the electrode structure 300 in adirection in which the electrodes 110 and 120 and the separator 130 arestacked (D in FIG. 1 ). When the outermost surface in one direction ofthe stacking directions is referred to as a top surface 300 a and theoutermost surface in the other direction is referred to as a bottomsurface 300 b, the metal substrate 220 may be disposed on the topsurface 300 a or the bottom surface 300 b. Specifically, referring toFIG. 1 , in the direction in which the electrodes 110 and 120 and theseparator 130 are stacked, the separator 130 is disposed at an outermostpart of the electrode structure 300, and the metal substrate 220 may bedisposed on the separator 130 which is disposed at the outermost part.Accordingly, an electrical short between the metal substrate 220 and theelectrode structure 300 may be prevented.

The metal substrate 220 plays a role in supplying lithium into theelectrode structure during the pre-lithiation process.

The metal substrate 220 may include a lithium layer 202. The lithiumlayer 202 plays a role in supplying lithium to the positive electrode120 in an embodiment of the present invention.

The lithium layer 202 may contain lithium, and may be specificallyformed of lithium. The lithium layer 202 may be in contact with theseparator disposed at the outermost part of the electrode structure 300.

A thickness of the lithium layer 202 may be in a range of 2.5 μm to 300μm, and may be specifically in a range of 10 μm to 150 μm. In a case inwhich the above range is satisfied, since the lithium layer may nolonger exist after the pre-lithiation is completed, stability may beimproved when the battery is operated.

The metal substrate 220 may further include a support 201. Specifically,the metal substrate 220 may include the support 201 and the lithiumlayer 202 disposed on the support 201. The support 201 is used tosupport the lithium layer 202 containing lithium in an amount requiredfor the pre-lithiation.

3. First Step and Second Step

The method of preparing a secondary battery of the embodiment mayinclude: a first step of applying a first current by electricallyconnecting the positive electrode and the negative electrode; and asecond step of applying a second current by electrically connecting themetal substrate and the positive electrode, after the first step. Boththe first step and the second step may be performed in a state in whichthe electrode assembly is impregnated with an electrolyte.

The electrolyte may include an organic liquid electrolyte, an inorganicliquid electrolyte, a solid polymer electrolyte, a gel-type polymerelectrolyte, a solid inorganic electrolyte, or a molten-type inorganicelectrolyte which may be used in the preparation of the lithiumsecondary battery, but the present invention is not limited thereto.

Specifically, the electrolyte may include a non-aqueous organic solventand a metal salt.

As the non-aqueous organic solvent, for example, an aprotic organicsolvent, such as N-methyl-2-pyrrolidone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,γ-butyrolactone, 1,2-dimethoxy ethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide,diemthylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphate triester, trimethoxy methane, adioxolane derivative, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, atetrahydrofuran derivative, ether, methyl propionate, and ethylpropionate, may be used.

In particular, since ethylene carbonate and propylene carbonate, ascyclic carbonates among the carbonate-based organic solvents, are highlyviscous organic solvents and have high dielectric constants, theethylene carbonate and propylene carbonate may well dissociate a lithiumsalt, and, thus, the ethylene carbonate and propylene carbonate may bepreferably used. Since an electrolyte having high electricalconductivity may be prepared when the above cyclic carbonate is mixedwith low viscosity, low dielectric constant linear carbonate, such asdimethyl carbonate and diethyl carbonate, in an appropriate ratio, theethylene carbonate and propylene carbonate may be more preferably used.

A lithium salt may be used as the metal salt, and the lithium salt is amaterial that is easily soluble in the non-aqueous electrolyte, wherein,for example, at least one selected from the group consisting of F⁻, Cl⁻,I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻,(CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻,(FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻,CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and (CF₃CF₂SO₂)₂N⁻ may be usedas an anion of the lithium salt.

In order to improve the life characteristics of the battery, suppressthe reduction in battery capacity, and improve discharge capacity of thebattery, at least one additive, for example, a halo-alkylenecarbonate-based compound such as difluoroethylene carbonate, pyridine,triethylphosphite, triethanolamine, cyclic ether, ethylenediamine,n-glyme, hexaphosphorictriamide, a nitrobenzene derivative, sulfur, aquinone imine dye, N-substituted oxazolidinone, N,N-substitutedimidazolidine, ethylene glycol dialkyl ether, an ammonium salt, pyrrole,2-methoxy ethanol, or aluminum trichloride, may be further included inthe electrolyte in addition to the electrolyte components.

The secondary battery may further include a battery case. Specifically,the electrode assembly and the electrolyte may be disposed in thebattery case, and the electrode assembly in the battery case may beimpregnated with the electrolyte.

(1) First Step

Referring to FIG. 1 , in the first step, the positive electrode 120 andthe negative electrode 110 are electrically connected (C1), and a firstcurrent may be applied. Since lithium ions of the positive electrode 120may be supplied to the negative electrode 110 by the first current, thelithium ions may be intercalated into the negative electrode.Accordingly, since irreversible sites in the negative electrode may befilled with the lithium ions, initial efficiency of the secondarybattery may be improved. Also, since a desired level of lithium ions maybe rapidly transferred from the positive electrode to the negativeelectrode due to a large potential difference between the positiveelectrode and the negative electrode, processability may be improvedduring the preparation of the secondary battery. Furthermore, since eachof the plurality of negative electrodes is mainly supplied with lithiumions from the adjacent positive electrode, a uniform amount of thelithium ions may be intercalated into the negative electrodes in thefinal electrode assembly in which the pre-lithiation is completed andthicknesses of the negative electrodes may be uniform. Accordingly, thedistortion and disconnection of the battery may be suppressed.

Specifically, in the first step, the positive electrodes 120 areconnected to each other to constitute a positive electrode connectedbody (not shown), and the positive electrode connected body (not shown)is connected to a positive electrode lead (not shown). The negativeelectrodes 110 are connected to each other to constitute a negativeelectrode connected body (not shown), and the negative electrodeconnected body (not shown) is connected to a negative electrode lead(not shown). Thereafter, the positive electrode lead (not shown) and thenegative electrode lead (not shown) may be electrically connected (C1).

The first current may be supplied in a constant current mode.

An amount of the first current may be in a range of 0.001 C to 1.000 C,particularly 0.010 C to 0.500 C, and more particularly 0.050 C to 0.200C. When the amount of the first current is less than 0.001 C, processtime is excessively increased to reduce the processability, and, whenthe amount of the first current is greater than 1.000 C, it is difficultto perform uniform charge due to a rapid reaction of the electrode.Herein, 1 C means an amount of current which may charge or discharge abattery in one hour.

The first step may be performed from a state of charge (SOC) of 1% to anSOC of 50%, for example, from an SOC of 5% to an SOC of 30%. Theexpression “SOC %” refers to a degree of charge of the secondarybattery, wherein SOC 0% means a fully discharged state, and SOC 100%means a fully charged state. In a case in which the above range issatisfied, irreversible capacity of the negative electrode iscompensated to an appropriate level to increase the capacity of thebattery, and cycle life may be improved due to excess lithium in thenegative electrode.

(2) Second Step

Referring to FIG. 2 , in the second step, the positive electrode leadand the negative electrode lead are disconnected, the positive electrodelead (not shown) and the metal substrate 220 are electrically connected(C2), and a second current may be applied. Since lithium ions of themetal substrate 220 may be supplied to the positive electrode 120 by thesecond current, the lithium ions may be intercalated into the positiveelectrode. Accordingly, the lithium ions deintercalated from thepositive electrode in the first step may be compensated from the metalsubstrate 220. Also, since a desired level of lithium ions may berapidly transferred from the metal substrate 220 to the positiveelectrode 120 due to a large potential difference between the positiveelectrode and the metal substrate, the processability may be improvedduring the preparation of the secondary battery.

Specifically, in the second step, each of the positive electrode 120 andthe metal substrate 220 includes a tab (not shown), the tabs of theplurality of positive electrodes 120 are connected to each other, and,thereafter, the tabs connected to each other and the metal substrate 220may be electrically connected (C2).

In the second step, the second current is applied in a constant currentand constant voltage mode, and an amount of the current applied in theconstant current mode may be in a range of 0.0001 C to 0.1 C, forexample, 0.001 C to 0.05 C. In a case in which the above range issatisfied, uniform pre-lithiation may be achieved within a reasonableprocess time.

The secondary battery prepared according to the embodiment of thepresent invention is suitable for portable devices, such as mobilephones, notebook computers, and digital cameras, and electric cars suchas hybrid electric vehicles (HEVs), and, particularly, may be preferablyused as a component battery of a medium and large sized battery module.Also, the above-described secondary battery may be included as a unitcell in a medium and large sized battery module. The medium and largesized battery module may be preferably used as a power source requiringhigh power and large capacity, for example, an electric vehicle, ahybrid electric vehicle, or a power storage device.

Hereinafter, examples of the present invention will be described indetail in such a manner that it may easily be carried out by a personwith ordinary skill in the art to which the present invention pertains.The invention may, however, be embodied in many different forms andshould not be construed as being limited to the examples set forthherein.

EXAMPLES AND COMPARATIVE EXAMPLES Example 1: Preparation of SecondaryBattery

(1) Preparation of Electrode Assembly and Electrolyte Impregnation

1) Preparation of Electrode Assembly

A negative electrode including a negative electrode collector and anegative electrode active material layer disposed on both sides of thenegative electrode collector was prepared. The negative electrode activematerial layer contained silicon particles, PAA as a binder, and carbonblack, as a conductive agent, in a weight ratio of 75:10:15, wherein aloading amount was 10.4 mAh/cm². Also, although similar to the abovenegative electrode, a single-sided negative electrode, in which thenegative electrode active material layer was disposed only on one sideof the negative electrode collector, was prepared.

A positive electrode including a positive electrode collector and apositive electrode active material layer disposed on the positiveelectrode collector was prepared. The positive electrode active materiallayer contained LiMn₂O₄ having an average particle diameter (D₅₀) of12.5 μm, PVDF as a binder, and CNT, as a conductive agent, in a weightratio of 95:1.5:3.5, wherein a loading amount was 5 mAh/cm².

Both the negative electrode and the positive electrode included holeshaving a diameter of 10 μm, and a ratio of an area of a surface of thenegative electrode (or positive electrode) to an area occupied byopenings of the holes on the surface of each individual negativeelectrode (or positive electrode) was 99:1. Also, a spacing between theholes was 100 μm.

After stacking the five negative electrodes and the five positiveelectrodes with a polyolefin-based separator disposed therebetween, thesingle-sided negative electrode was disposed on a top layer (see FIG. 1). Thereafter, separators were disposed on an uppermost surface and alowermost surface, respectively.

Thereafter, a metal substrate was disposed on the uppermost surface. Themetal substrate included a 150 μm thick lithium layer (formed oflithium) and a copper support, and the lithium layer was disposed to bein contact with the separator constituting the uppermost surface.

2) Electrolyte Impregnation

After putting the electrode assembly in a case, an electrolyte, in which1.0 M lithium hexafluorophosphate (LiPF₆) was dissolved in an organicsolvent composed of fluoroethylene carbonate/ethylmethyl carbonate(mixing volume ratio of FEC to EMC=3:7), was injected into the case toimpregnate the electrode assembly with the electrolyte. From this, apreliminary battery was prepared.

(2) First Step

In the preliminary battery, five positive electrode tabs were connectedto each other to form a positive electrode connected body, and thepositive electrode connected body was connected to a positive electrodelead. Also, the five negative electrodes and one single-sided negativeelectrode were connected to each other to form a negative electrodeconnected body, and the negative electrode connected body was connectedto a negative electrode lead. Thereafter, the positive electrode leadand the negative electrode lead were electrically connected.

Thereafter, the preliminary battery was charged to an SOC of 10% byapplying a first current at 0.1 C for 1 hour.

(3) Second Step

After the first step, the positive electrode lead and the negativeelectrode lead were disconnected, and the metal substrate and thepositive electrode lead were then connected. Thereafter, a current wasapplied at 0.002 C-3.0 V (CC-CV) for 160 hours to supply lithium ions ofthe lithium layer of the metal substrate to the positive electrode.

Comparative Example 1: Preparation of Secondary Battery

After preparing a preliminary battery in the same manner as in Example1, a negative electrode lead and a metal substrate were electricallyconnected, and a current was applied at 0.002 C-0.2 V (CC-CV) for 180hours to supply lithium ions of a lithium layer of the metal substrateto a negative electrode until SOC reached 4%.

Experimental Example 1: Check Time to Achieve Pre-lithiation Target (SOC10%)

With respect to Example 1 ((A) of FIG. 4 ), it took 1 hour in the firststep and 160 hours (CC: 40 hours, CV: 120 hours) in the second step toachieve an SOC of 10%, wherein it took a total of 161 hours. Withrespect to Comparative Example 1 ((B) of FIG. 4 ), it took a total of180 hours to achieve an SOC of 4%, and, since it was considered that itwas not possible to reach an SOC of 10% from a current and SOC % trend,the experiment was terminated.

Experimental Example 2: Confirmation of Increase in Thickness of theNegative Electrode after Pre-Lithiation

2^(nd) in FIG. 5 refers to the negative electrode closest to the metalsubstrate except for the single-sided negative electrode, andthereafter, the negative electrodes were located away from the metalsubstrate in the order of 3^(rd), 4^(th), 5^(th), and 6^(th). Withrespect to Example 1 ((A) of FIG. 5 ), increases in thickness of thenegative electrodes confirmed after the completion of the pre-lithiationwere almost similar to each other. In contrast, with respect toComparative Example 1 ((B) of FIG. 5 ), an increase in thickness of the2nd negative electrode, which was closest to the metal substrate, wasthe largest excluding the single-sided negative electrode, and increasesin thickness of the remaining negative electrodes were less than that.That is, with respect to Comparative Example 1, it may be understoodthat amounts of lithium intercalated into the negative electrodes werenot uniform.

1. A method of preparing a secondary battery, the method comprising:pre-lithiating an electrode assembly comprising an electrode structurecomprising a plurality of electrodes and a plurality of separators; anda metal substrate, wherein the plurality of electrodes and the pluralityof separators are alternatingly stacked, wherein the metal substrate ispresent on an outermost surface of the electrode structure in adirection in which the electrode and the separator are stacked, whereinthe plurality of electrodes comprises negative electrodes, eachcomprising negative electrode active material layer, and positiveelectrodes, each comprising positive electrode active material layer,wherein each positive electrode and negative electrode are spaced apartfrom each other with one separator of the plurality of separatorsdisposed therebetween, wherein the pre-lithiating comprises: applying afirst current by electrically connecting one of the plurality ofpositive electrodes and one of the plurality of negative electrodes; andapplying a second current by electrically connecting the metal substrateand one of the plurality of positive electrodes, after applying thefirst current.
 2. The method of claim 1, wherein an amount of the firstcurrent is in a range of 0.001 C to 1.000 C.
 3. The method of claim 1,wherein the first step is performed from a state of charge (SOC) of 1%to an SOC of 50%.
 4. The method of claim 1, wherein, the second currentis applied in a constant current and constant voltage mode, and anamount of the current applied in the constant current mode is in a rangeof 0.0001 C to 0.1 C.
 5. The method of claim 1, wherein the plurality ofelectrodes comprise holes penetrating the individual electrodes.
 6. Themethod of claim 5, wherein a ratio of an area of a surface of eachindividual electrode to an area occupied by openings of holes on thesurface of each individual electrode is in a range of 99.9:0.1 to 80:20.7. The method of claim 5, wherein an opening of each hole has a diameterof 1 μm to 100 μm.
 8. The method of claim 1, wherein the negativeelectrode active material layer comprises a negative electrode activematerial, and wherein the negative electrode active material comprisessilicon.
 9. The method of claim 1, wherein the metal substrate comprisesa lithium layer.
 10. The method of claim 9, wherein the lithium layerhas a thickness of 2.5 μm to 300 μm.